There is known a camera system that enables, by using a camera mounted on a rear side of a vehicle to display an image behind the vehicle where it is hard for a driver to view on a display unit inside the vehicle (for example, a display unit of a vehicle navigation device). For such a system, there is known a method, by adjusting a difference between an assumed image capturing a subject (index) whose position information is known and an actual image of the index actually taken by the camera, to obtain parameters for calibration of a camera mounting position (for example, see Patent Document 1 and Non-Patent Document 1).